Isogamy

Isogamy is a form of sexual reproduction that involves gametes of similar morphology (similar shape and size), differing in general only in alleleexpression in one or more mating-type regions. Because both gametes look alike, they cannot be classified as "male" or "female." Instead, organisms undergoing isogamy are said to have different mating types, most commonly noted as "+" and "−" strains, although in some species there are more than two mating types (designated by numbers or letters). Fertilization occurs when gametes of two different mating types fuse to form a zygote.

Evolution

It appears that isogamy was the first stage of sexual reproduction. In several lineages (plants, animals), this form of reproduction independently evolved to anisogamous species with gametes of male and female types to oogamous species in which the female gamete is very much larger than the male and has no ability to move. There is a good argument that this pattern was driven by the physical constraints on the mechanisms by which two gametes get together as required for sexual reproduction.

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